Biocompatible and bio-absorbable sutures have been widely used for soft tissue approximation for many years. In addition to the requirement of good biocompatibility when implanted in human patients, there are a number of other characteristics that are very important and critical to surgeons and patients. Some of the most important characteristics for a bio-absorbable suture include, but are not limited to, tensile strength, in vivo breaking strength retention, elongation at break, knot tensile strength, in vivo absorption rate and softness.
Various processes, which include melt extrusion and drawing orientation (i.e., spinning and braiding), are currently used to make bio-absorbable multifilament surgical sutures from a copolymer containing glycolide (PGA) and lactide (PLA). Although such processes generally produce PGA/PLA sutures having many of the above-mentioned characteristics within preferred ranges, especially a relatively fast absorption rate (approximately 60-90 days for near complete absorption), they have a relatively low tensile strength compared to nonabsorbable fibers such as nylon or polyester.
Tensile strength is a measure, prior to implantation of the suture braid in a patient, of the amount of tension that a fiber or suture can withstand before it breaks. If the fiber tensile strength is being measured, it is known as the fiber tenacity. The fiber tenacity achieved by processes that produce PGA/PLA sutures is typically in the range of from approximately 6.0 and 6.8 grams per denier (g/d) and sometimes up to 7.2 g/d. Any increase in these tenacity values that could be achieved without diminishing the other characteristics of the suture would be important and useful. In vivo strength retention is a measure of the strength possessed by the suture braid after the suture has been implanted in a patient. Elongation at break is also referred to simply as elongation and is a measure of how much elongation of the suture fibers occurs prior to breakage upon application of tension. It is preferable to maintain the fiber elongation between approximately 22% and 35%.
Various attempts have been made to produce PGA/PLA sutures having a higher tensile strength, while remaining within the preferred ranges for the other desired characteristics, including bio-absorbability and elongation. For example, some suture manufacturers have tried putting more fibers into a braid of a given suture size. Although a higher tensile strength of the overall braid may be obtained in this manner, either the resulting suture would have to be highly oversized or the braid must be tightly packed, which could yield sutures of diminished handling characteristics, such as increased stiffness and poor knot security.
A better way to obtain PGA/PLA sutures having higher tensile strength is to increase the fiber tenacity (measured as force per unit titer), which will yield higher tensile strength for the braid without requiring an increase in the total number of fibers in a braid. For example, U.S. Pat. Nos. 5,585,056 and 6,005,019 disclose the use of plasticizers as a process aid to improve multifilament yarn drawability and the properties of the fibers made from a copolymer having 92.5:7.5 molar ratio poly(glycolide-co-lactide). The plasticizer may have helped to lower the melting point of the copolymer, thereby allowing extrusion without melt fracture at relatively low temperatures. The highest tensile strength obtained by the processes disclosed in these patents was 7.2 grams/denier (g/d), but the elongation at break dropped to 21% and less. This low elongation may lead to severe filament breakage and operational difficulties in the downstream processing of the fibers, including twisting and braiding the fibers together to make a braided suture or other surgical articles. The suture handling characteristics could also be compromised if the fiber elongation at break is too low.
U.S. Pat. No. 6,277,927 discloses that better in vivo strength retention may be achieved by using block copolymers of PGA/PLA to make the suture fibers. However, the fibers spun from such block copolymers failed to exhibit high initial fiber and suture strength. RU 2,073,074 discloses the making of suture fibers by forcing a PGA/PLA copolymer melt to pass through a very thin channel in the spin pack. It was believed that more uniform heating of the fibers could be obtained by the aforesaid method such that better productivity and better fiber properties could be obtained. The maximum fiber tensile strength obtained by the method of RU 2,073,074 for a PGA/PLA copolymer, however, was only about 6.0 to 6.4 g/d. In U.S. Pat. No. 5,288,516, a method is disclosed to make a high tensile strength fiber from PGA, however, sutures made of only PGA fibers have a significantly increased absorption time, which is an undesirable characteristic in many cases where soft tissue approximation is required.
The device and process of the present invention address the shortcomings of the existing apparatus and processes for manufacturing absorbable suture fibers.
As will be described in further detail hereinafter, the present invention introduces modifications to the equipment, temperature profile and heat retention aspects of the known process and equipment.